1. Field of the Invention
This invention relates to electrographic processes for making images. More particularly, this invention relates to electrographic processes and the elements used therein for the production of images on substrates that are not readily amenable to direct electrographic imaging.
2. Description of Related Art
The use of electrographic processes to generate images, including multicolor images, is well known in the art. In such processes, a latent image in the form of a distribution of electric charges is produced directly on a substrate having a dielectric surface using an electrographic printer. The printer operates by depositing charge imagewise onto the dielectric surface of the substrate using a scanning stylus or a plurality of styli are arranged in linear arrays across the width of the moving dielectric surface to create charge patterns.
Color images may be generated using a plurality of serially positioned charge depositing and toning stations which operate sequentially to apply three or four colors on a moving web to generate a colored image thereon. U.S. Pat. No. 4,569,584 discloses apparatus for color image reproduction which uses only one stylus array with toning stations located on either side of this array; the accepting surface web is traversed to-and-from to make successive images using different color toners.
Electrographic images are inherently of low gloss. There are two reasons for this. Firstly, the technology the toners which form the image retain most of their shape and unity on drying, rather than flowing and showing a smooth surface. This prevents the image from gaining more than a slight degree of gloss. Additionally, the surface of the image receptive layer must itself be a matte surface for the generation of the image.
The toned surface is exposed and thus vulnerable to damage in handling and viewing. Protective coatings for images are often used to give resistance to abrasion, UV light degradation, bacterial degradation, molds and fungi, unwanted markings especially graffiti or to chemical degradation from water, smog or other chemical agent.
Together, these factors lower the aesthetic and practical value of the prints and induce a very high proportion of the practitioners of the art to apply an additional coating by lamination or spray to the electrographic image, which is an extra processing step, and which adds cost.
Another difficulty for the electrographic printing industry is that there are many substrates upon which it is desirable to print. Many of these could conceivably be manufactured in forms suitable for direct electrographic imaging but their development or manufacture is uneconomical and hence they are either expensive or outright unavailable. Examples of this include colored papers and films, and electrographic imageable fabrics. Then there are those which because of their physical properties (bulk, stiffness, low strength, elasticity, or structure) can not be transported through a printer and hence are completely unsuited for electrographic imaging. Thick films, papers and boards; wooden, ceramic and metal surfaces are but a few examples. The practitioners of the art will quickly identify more. The ability to provide images on such substrates is desirable.
The generation of images on substrates by first generating the image on a highly transparent electrographic element and then adhering the complete element to a substrate using a suitable adhesive agent is practiced in the art. The deficiencies of this method are that to obtain high transparency the electrographic element must comprise a film carrier and image quality on electrographic elements in which the carrier is film have generally lower image quality than those where the carrier is paper. Films are also more costly than papers.
A method for transferring a fixed image, such as an electrographically produced toner image, from an initial substrate to a final substrate is disclosed in U.S. Pat. No. 4,983,487. The disclosed method employs an adhesive-coated film to lift the toned image from its initial substrate and to secure it to the final substrate. The film remains in place after the transfer is completed and serves to encapsulate and protect the image. The initial substrate remains intact and may be reused.
A transfer process is disclosed in U.S. Pat. No. 5,102,768 for providing a non-electrostatically transferred toned image. In this process, an electrostatic latent image is conventionally formed on the surface of an element and that element is conventionally developed into a visible image by applying toner powder. The toned image is then thermally transferred from the surface of an element by contact to the face of a thermoplastic film that is strippably laminated to a paper or like backing. The film is then positioned against a receiver with the toner image therebetween, and the composite is subjected to two successive stages of compressive heating. It is disclosed that the process produces high resolution images from very small particle size toner powder on rough paper.
An offset transfer process of electrographically produced toner images is disclosed in U.S. Pat. No. 5,108,865. In the disclosed process, a liquid toned image is generated on the surface of an electrographic element. The image is adhered to the adhesive surface of a temporary receptor sheet which comprises a carrier layer, releasable release layer, and a transferable adhesive layer secured to the release layer. The temporary receptor sheet with the image adhered thereto is removed from the electrographic element, and then the image surface of the temporary receptor sheet is contacted with a final receptor surface. The adhesive layer secures the toner image, adhesive layer and release layer to the final receiving layer and the carrier layer is removed from the release layer to generate the final image wherein the release layer now is a top protective layer.
The electrographic processes disclosed in the patent publications discussed supra, employ a transfer of the toned image from an electrographic element to the final substrate using an intermediate transfer element. Although advances have been made in retaining the integrity of the toned image, such transfer steps remain prone to image degradation by abrasion or chemical interaction unless added laminating or coating steps are used.
There continues to be a need for a simplified process to provide protected, distortion-free, full-color images, particularly, for use on large format posters, billboards and the like.